Tire air pressure optimization system for improving tread wear

ABSTRACT

A system is provided for automatically adjusting the air pressure of a tire based on variations in loading to improve tread wear. For a particular loading of the tire, the system provides for the determination of a preferred air pressure setting designed to minimize tread wear. This preferred air pressure, P p , is determined from an analysis of the longitudinal stresses that occur within the contact patch of the tread.

FIELD OF THE INVENTION

The present invention relates to a system for automatically increasingor decreasing the air pressure of a tire based on the changes in itsloading so as to improve the wear of the tread.

BACKGROUND OF THE INVENTION

Commercial vehicles typically transport loads that can varysubstantially in amount between different loads. When maintained at arelatively constant air pressure, the tire on such a vehicle willundergo changes to the shape of the contact patch as the loading isvaried. As a result, the wear life of the tire's tread will be adverselyaffected.

Systems have been developed that allow for the addition of air to a tirein order to maintain a preset pressure. For example, commercial vehiclessuch as heavy duty truck and trailer combinations may be equipped withan air source and pressurized air storage systems for adding air to oneor more tires if the measured air pressure falls below a predeterminedvalue. In addition, systems have been provided for determining the loadon a given axle based upon, e.g., the air spring operating pressure.However, none of these systems provide for automatically increasing anddecreasing tire air pressure depending upon changes in the loading ofthe tire as the vehicle is used, much less provide for adjusting thepressure of the tires to a specific pressure based on loading in orderto optimize the life of the tread.

In addition to variations in loads, the construction and positioning oftires used on commercial vehicles can also vary substantially.Commercial tires come in a variety of constructions and can includee.g., ribbed and non-ribbed tread. For a tractor and trailercombination, tire positions can include steer, drive, tag, and trailerpositions. The impact on tread wear due to variations in loads willlikely not be uniform between tires of different construction or betweentires at different positions on a commercial vehicle. As a result, withload variations, the changes in air pressure required to minimizechanges in the contact patch and improve tread wear will likely varybetween the different tires and/or positions on the vehicle.

Manually adjusting tire air pressure based on changes in loading is notpractical for a variety of reasons. Commercial vehicles may include alarge number of tires; a well known example in North America includesthe eighteen tires of the frequently used heavy duty tractor and trailercombinations. Manually inflating or deflating the trailer and/or drivetires each time a loading of the vehicle occurs would be time consumingand impractical.

A practice commonly followed by commercial vehicle operators is toemploy a single, predetermined air pressure for all loads regardless ofthe variations between the loads. For example, a fleet operator mayattempt to maintain all trailer tires above a certain minimum pressuresetting but within the maximum pressure limit for a particular tire. Theminimum pressure setting can be selected based on the minimum pressurerecommended by e.g., guidelines provided by the Tire and Rim Associationfor the maximum load expected to be applied to the tire. Unfortunately,as previously stated, this constant approach is deleterious to treadlife as changes in the load will cause variations in the contact patch.In addition, aside from the above described problems of a constantpressure approach, the pressure recommendations provided by the Tire andRim Association are not based upon the optimization of tread wear for agiven tire loading and do not suggest how such pressures might bedetermined.

Accordingly, a system for automatically adjusting the air pressure of atire based on changes in loading is needed. More particularly, a systemthat improves tread wear performance by automatically adjusting the airpressure to a preferred air pressure as variations in loading of thetire occur would be very useful.

SUMMARY OF THE INVENTION

Aspects and advantages of the invention will be set forth in part in thefollowing description, or may be obvious from the description, or may belearned through practice of the invention.

In one exemplary aspect, a method for regulating the air pressure in atire is provided. The tire includes a tread having a ground contactingportion and defines lateral and longitudinal directions. The methodincludes dividing the ground contacting portion of the tread intomultiple zones along the lateral direction, the multiple zones includinga shoulder zone along each shoulder of the tire and one or moreintermediate zones between the shoulder zones; selecting a range of tireair pressures and a range of tire loads; determining, for a load L_(i)in the selected range of tire loads, the average longitudinal stressσ_(x) in each of the zones over the selected range of tire airpressures; identifying, from the selected range of tire air pressures, apreferred air pressure P_(p) for the load L_(i), wherein the preferredair pressure P_(p) is the pressure at which both i) the averagelongitudinal stress σ_(x-avg) for all of the zones is minimized and ii)the average longitudinal stress σ_(x-avg) for each of the intermediatezones is maintained at or above a predetermined threshold longitudinalstress σ_(x-th); repeating, over the selected range of tire loads L_(i),the steps of determining and identifying so as to provide a correlationof the preferred air pressures P_(p) corresponding to multiple loadsL_(i) for the selected range of tire loads; ascertaining an actual oranticipated tire load L_(a) on a vehicle; and adjusting tire airpressure during vehicle use to the preferred air pressure P_(p) for thetire load L_(a) using the correlation from the step of repeating.

For the load L_(i), the step of identifying may include eliminating fromthe selected range of air pressures all pressures at which the averagelongitudinal stress σ_(x-avg) for any zone is less than the thresholdlongitudinal stress σ_(x-th) and, if there is no pressure at which theaverage longitudinal stress σ_(x-avg) for any zone is less than thethreshold longitudinal stress σ_(x-th), then eliminating from theselected range of air pressures all pressures at which the averagelongitudinal stress σ_(x-avg) for either of the shoulder zones is lessthan the threshold longitudinal stress σ_(x-th); calculating S, the sumof the squares of the average longitudinal stress σ_(x-avg), of eachzone, for each of the pressures remaining in the selected range of airpressures after the step of eliminating; and selecting, from thepressures remaining in the selected range of air pressures after thestep of eliminating, the preferred air pressure P_(p) as the greater ofeither a) the air pressure having the smallest value of S or b) the airpressure that is the minimum air pressure P_(min) recommended for theload L_(i).

This exemplary method may further include identifying the highest loadL_(Pmin-lowest) for which the preferred pressure P_(p) is equal to theminimum pressure desired P_(min-lowest) for the tire and, if such a loadexists, then selecting the minimum pressure desired P_(min-lowest) asthe preferred pressures P_(p) for all loads below the loadL_(Pmin-lowest).

The shoulder zones can be defined, for example, to include shoulder ribslocated along the shoulders of the tire. Each of the one or moreintermediate zones can be defined, for example, as one of the ribslocated between the shoulder ribs of the tire. The selected range of airpressures can be defined as the pressures between about 5 psi less thanthe minimum air pressure desired, P_(min-lowest), for the tire to aboutthe maximum sidewall pressure for the tire. The selected range of loadscan be defined as the loads between about the minimum operating load ofthe tire to about the maximum sidewall load of the tire.

The step of determining the average longitudinal stresses σ_(x) caninclude measuring longitudinal stresses σ_(x) in one or more of thezones. Alternatively, the step of determining the average longitudinalstresses σ_(x) can include calculating the longitudinal stresses σ_(x)in one or more of the zones. The step of determining the averagelongitudinal stresses σ_(x) can include measuring or calculating the netforce of contact per zone and dividing by the area of the zone incontact with the ground.

The step of adjusting the tire pressure can include increasing the airpressure, decreasing the air pressure, or both. The step of ascertainingthe actual load can include measuring the load carried by the vehicleor, alternatively, the air pressure can be adjusted based on ananticipated tire load. This exemplary method of the invention mayinclude measuring tire air pressure during vehicle use before the stepof adjusting.

The present invention applies to different types of tires and differentpositions on a vehicle as well. For example, the tire may be anon-ribbed tire or a ribbed tire. By way of further example, the tiremay be used on a trailer and a threshold longitudinal stress σ_(x-th) inthe range of about −0.10 bar to about −0.12 bar is typical for manyapplications. Drive tires, in contrast, see a significant longitudinalstress σ_(drive) coming from the drive torque of the motor; it is thusappropriate to offset the trailer tire threshold by this amount for thedrive tires. The range of σ_(drive) varies significantly depending onengine, load, and other factors. As another example, the tire may beused on a tractor in the steer position and a threshold longitudinalstress σ_(x-th) in the range of about −0.04 bar to about −0.06 bar istypical for this application.

These and other features, aspects and advantages of the presentinvention will become better understood with reference to the followingdescription and appended claims. The accompanying drawings, which areincorporated in and constitute a part of this specification, illustrateembodiments of the invention and, together with the description, serveto explain the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including thebest mode thereof, directed to one of ordinary skill in the art, is setforth in the specification, which makes reference to the appendedfigures, in which:

FIG. 1 illustrates the results of an exemplary determination oflongitudinal stress as calculated for each rib of an XOne® XTE tire overa range of pressures for a given tire loading.

FIG. 2 illustrates a preferred pressure P_(p) curve and a P_(min) curveover a range of loads for an XOne® XTE tire as further described below.Pressures recommended by the Tire and Rim Association are alsoillustrated.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a system for automatically adjustingthe air pressure of a tire based on variations in loading to improvetread wear. For a particular loading of the tire, the system providesfor the determination of a preferred air pressure setting designed tominimize tread wear. This preferred air pressure, P_(p), is determinedfrom an analysis of the longitudinal stresses that occur within thecontact patch of the tread as more fully described below. For purposesof describing the invention, reference now will be made in detail toembodiments and aspects of the invention, one or more examples of whichare illustrated in the drawings. Each example is provided by way ofexplanation of the invention, not limitation of the invention. In fact,from the teachings disclosed herein, it will be apparent to thoseskilled in the art that various modifications and variations can be madein the present invention without departing from the scope or spirit ofthe invention. For instance, features illustrated or described as partof one embodiment, can be used with another embodiment to yield a stillfurther embodiment. Thus, it is intended that the present inventioncovers such modifications and variations as come within the scope of theappended claims and their equivalents.

“Longitudinal” or “longitudinal direction” as used herein with referenceto the tire tread means a direction perpendicular to axis of rotation ofthe tire, tangent to the circumferential direction of tread, andparallel to the ground. The use of “x” or “x-direction” is also used torefer to the longitudinal direction.

“Lateral” as used herein refers to a direction parallel to the axis ofrotation of the tire and perpendicular to the longitudinal direction.

“Longitudinal stress” as used herein refers to the mechanical stressoccurring in the tire tread as determined by dividing the area of thetread in contact with the ground (or a zone/section thereof) by the netforce acting on such area along the x or longitudinal direction at agiven air pressure and loading of the tire. As will be understood by oneof ordinary skill in the art, the amount of longitudinal stress can bedetermined through measurement or modeling using e.g., finite elementanalysis. The average longitudinal stress within a given zone of thetread will be referred to herein with “σ_(x-avg)” as is more fullydiscussed below. Positive values of σ_(x-avg) denote the direction oftravel as the tire moves along the ground or surface.

P_(min) as used herein refers to the minimum air pressure recommendedfor the tire for a given loading. The Tire and Rim Association providesguidelines indicating the minimum air pressure that should be used foreach specific tire.

P_(min-lowest) as used herein is defined as the lowest pressure desiredfor the tire in question. P_(m-lowest) is determined by factors otherthan tire wear performance and typically includes tire reinflation time,tire lateral firmess, and other considerations. It is always greaterthan or equal to P_(min).

As previously stated, a load-induced variability in the size of a tire'scontact patch will contribute to unfavorable tread wear. The size of thecontact patch is also directly affected by the amount of air pressure inthe tire. To improve tread wear, load-induced variations in the size ofthe contact patch can be controlled by adjusting the tire air pressurewithin the minimum and maximum limits of air pressure for a particulartire and its loading. More importantly, as will now be described byexamples, the present invention provides a system for determining apreferred pressure P_(p) for optimizing tread wear at a given tireloading. As the loading of a tire changes based on e.g., changes invehicle cargo, tire air pressure can be automatically adjusted (bydeflation or inflation) to the preferred air pressure P_(p) forminimizing tread wear for a particular construction, position, andloading of the tire.

According to an exemplary aspect of the present invention, the tread ofthe tire is divided into zones along the lateral direction. These zonescan be defined by e.g., features in the tread. For a ribbed tire, thezones can be divided among ribs located along the shoulders of the tireand the ribs that are intermediate thereof. For a non-ribbed tire, thetread pattern can be arbitrarily divided along the lateral directioninto zones so as to create different tread regions for purposes ofanalysis. By way of example, the tread region could be divided into ashoulder zone located along each shoulder of the tire and one or moreintermediate zones located between the shoulder zones. Preferably, thezones are of roughly equal width but variations can also be used.

Within each zone, the value and direction of the average longitudinalstress σ_(x-avg) provides a strong indicator of the wear potential forthe portion of the tread defined by the zone. Very high values of theaverage longitudinal stress typically lead to a rapid rate of tread wearwhile large negative values of the average longitudinal stress areassociated with a propensity for irregular tread wear. Accordingly, tominimize tread wear for a given construction, loading, and position ofthe tire, the air pressure should be modulated or adjusted so that thevalue of the average longitudinal stress σ_(x-avg) is both minimized andsimultaneously maintained above a threshold value of longitudinalstress, referred to herein as σ_(x-th). The tire air pressure at whichthis desired average longitudinal stress σ_(x-avg) occurs is thepreferred air pressure P_(p) for a given loading, position, andconstruction of the tire.

The value of threshold longitudinal stress σ_(x-th) that should beapplied will depend upon the design of the tire and the intendedapplication such as the tire position on a heavy duty, tractor andtrailer combination. The inventors have found that for heavy dutytrailer tires, a value in the range of about −0.10 bars to −0.12 bars istypical for many applications. Drive tires, in contrast, see asignificant positive longitudinal stress σ_(drive) coming from the drivetorque of the motor; it is thus appropriate to offset the trailer tirethreshold by σ_(drive) for the drive tires. The range of σ_(drive)varies significantly depending on engine, load, application and otherfactors. For steer tires on a heavy duty tractor, which see a verydifferent usage condition, a range of about −0.04 bars to 0.06 bars istypical for many applications. Using the teachings disclosed herein, itwill be understood that other ranges may be developed depending upon avariety of factors such as e.g., the intended application of the tire.

The value and direction of the average longitudinal stress σ_(x-avg) foreach zone of the tire can be determined by calculation or measurement aswill be understood by one of ordinary skill in the art. For purposes ofthis exemplary aspect of the present invention, the average longitudinalstress σ_(x-avg) for each zone of the tread is determined by measuringor calculating the net force of contact per zone and dividing by thearea of the zone that is in contact with the ground as the tire rotatesthrough the contact patch. It should be noted that for some designs, thetire may be provided with ribs or zones that are intended to besacrificial and specifically designed to have large values oflongitudinal stress. Such sacrificial cases should be excluded from thezones that are analyzed according to the system set forth herein.

For a given tire construction and position, the value of the preferredair pressure P_(p) will vary depending upon changes in the load that isapplied to the tire as e.g., the cargo of the vehicle is loaded orunloaded. To provide for automatic adjustment of the tire air pressure,a correlation is developed that relates the preferred air pressuresP_(p) with the range of loads anticipated for a particular tireconstruction and position. Using this correlation, the preferred airpressure P_(p) can be readily determined based upon knowledge of theactual or anticipated load L_(a) for the tire. The actual load L_(a) canbe readily determined using known on-board measurement systems such as,for example, currently available systems that determine weight based onmeasurements taken from the vehicle's suspension system. Knowing thispreferred pressure P_(p), the air pressure of the tire can be adjustedby inflating or deflating the tire until the preferred air pressureP_(p) is obtained for a specific loading L_(a).

The correlation is created by selecting a range of tire air pressuresand a range of tire loads that are anticipated and proper for the tirein question. For example, the selected range of loads may span from anunloaded tire up to the maximum loading allowed for the particularconstruction of the tire being considered. As will be understood by oneof skill in the art, for each tire model provided by manufacturers, amaximum loading for the tire is typically identified on the sidewall ofthe tire. This maximum limit can be selected as the upper end of theselected load range while an unloaded or zero load can be selected forthe lower limit. Other ranges may also be selected, provided such arewithin manufacturer and/or industry recommended limits.

Similarly, a pressure range for creation of the correlation is selected.For example, for purposes of creating the correlation, a pressure rangeof 5 psi below the minimum pressure desired P_(min-lowest) can be usedas the lower end of the range while the maximum air pressure recommendedby the Tire and Rim Association can be used for the upper end of therange. Other ranges may also be selected.

Next, over the selected range of air pressures and loads, the averagelongitudinal stress σ_(x) in each zone of the tire's tread isdetermined. More particularly, for a given load L_(i) within theselected range of tire loads, the average longitudinal stress σ_(x-avg)is a determined for each zone of the tire over the entire selectedpressure range. Suitable increments of pressure within the selected airpressure range may be used. For example, for a given load L_(i), theaverage longitudinal stress σ_(x-avg) can determined for each pressurebetween the minimum and maximum pressures in the selected air pressurerange using increments of 5 psi. Other increments may be used as well.

Using this stress data for the load L_(i), a preferred air pressureP_(p) from the selected air pressure range is identified for load L_(i).The preferred air pressure P_(p) is identified as the air pressure atwhich each of the following two conditions is simultaneously satisfied:i) the average longitudinal stress σ_(x-avg) for all zones is minimized;and ii) the average longitudinal stress σ_(x-avg) for each of theintermediate zones is maintained at or above a predetermined thresholdlongitudinal stress σ_(x-th).

In order to identify this preferred pressure P_(p) for the load L_(i),all air pressures that meet the following condition, referred to asCriterion 1, are eliminated from the selected air pressure range.

-   -   Criterion 1: The average longitudinal stress σ_(x-avg) for any        zone of the tread is less than the predetermined threshold        longitudinal stress σ_(x-th).        If at least one air pressure in the selected air pressure range        is not eliminated by Criterion 1, then for each of the remaining        non-eliminated air pressure(s), the sum of the squares of the        average longitudinal stress σ_(x-avg), S, is calculated for each        of the zones. This calculation is set forth in the following        equation:

$\begin{matrix}{S \equiv {\sum\limits_{zones}\sigma_{x}^{2}}} & (1)\end{matrix}$The preferred pressure P_(p) is selected as the greater of either a) theair pressure having the smallest value of S or b) the minimumrecommended air pressure P_(min) for the load L_(i). For example, as setforth above, the Tire and Rim Association provides a minimum recommendedair pressure P_(min) for specific tires based on the loading of thetire.

If all air pressures in the selected air pressure range would beeliminated by the application of Criterion 1, then the followingcondition, referred to as Criterion 2, is applied to all air pressuresin the selected air pressure range. More particularly, now all airpressures in the selected air pressure range that meet the followingcondition, referred to herein as Criterion 2, are eliminated from theselected air pressure range.

-   -   Criterion 2: The average longitudinal stress σ_(x-avg) for        either of the shoulder zones is less than the predetermined        threshold longitudinal stress σ_(x-th).        If at least one air pressure in the selected air pressure range        is not eliminated by Criterion 2, then for each of the        remaining, non-eliminated air pressure(s), equation 1 is used to        calculate S, the sum of the squares of the average longitudinal        stress σ_(x-avg), for each of the zones as set forth in        equation 1. The preferred pressure P_(p) is now selected as the        greater of either a) the air pressure having the smallest value        of 0 or b) the minimum recommended air pressure P_(min) for the        load L_(i).

If all air pressures in the selected air pressure range would beeliminated by Criteria 1 and 2, then for each of the air pressures inthe selected air pressure range, S is calculated for each of the zonesas set forth in equation 1. As before, the preferred pressure P_(p) isnow selected as the greater of either a) the air pressure having thesmallest value of Sorb) the minimum recommended air pressure P_(min) forthe load L_(i).

At this point, the correlation exists as a data set that contains thepreferred pressure P_(p) for a given load L_(i). To complete thecorrelation, the above described process of applying criterion 1 and 2over all pressures in the selected pressure range is now repeated forall loads in the selected range of loads. By way of example, preferablythe selected range of loads spans from the minimum operating load of thetire to the maximum load allowed for the tire. In order to determineP_(p) for this range of loads, suitable increments of tire load withinthe selected range of loads may used such as from 0 to 100 percent ofthe maximum load in increments of 10 percent. Other increments may beused as well.

Once Criteria 1 and 2 have been applied for all pressures in theselected pressure range and to all loads L_(i) within the selected rangeof loads, the correlation now contains a data set of the preferredpressures P_(p) for all the loads within the selected range of loadssubject to one additional modification. To finalize the correlation, thehighest load for which the preferred pressure P_(p) is equal to theminimum desired pressure P_(min-lowest) is identified. If such a loadexists (referred to herein as L_(Pmin-lowest)) then for all loads belowL_(Pmin-lowest) the preferred pressure P_(p) is set to the minimumpressure desired P_(min-lowest) for the tire. The correlation nowprovides a data set of the preferred pressures P_(p) for a range ofloads applied to the tire. Using the teachings disclosed herein, one ofskill in the art will understand that the correlation can be maintainedas a reference table, model, equation, or the like. Interpolationbetween values may also be applied.

Using the correlation of preferred pressures P_(p) and loads asdescribed above, the air pressure of a tire subjected to varying loadscan be readily adjusted based on changes in the load. After measuringthe actual load L_(a) (or predicting the anticipated load) on a tire andits actual air pressure, reference can be made to the correlation todetermine if the tire's air pressure is equal to the preferred airpressure P_(p) for the current load. If not, the tire can by inflated ordeflated as needed to match the tire's air pressure to the preferred airpressure P_(p). A correlation can be developed for each tireconstruction and position.

As stated, equipment is already available that can automatically measuretire air pressure and load while the tire is in use on the vehicle.Accordingly, by incorporating the correlation into an appropriateon-board controller as will be understood by one of ordinary skill inthe art using the teachings herein, a vehicle can be readily providedwith equipment for automatically adjusting tire air pressure as theloading is changed so as to improve tread wear performance.

By way of example, the above exemplary method was applied to the XOne®XTE, a commercial truck tire having nine ribs. A size of 445/50R22.5 anda trailer position were selected. However, as set forth above, it shouldbe understood that the present invention may be applied to all tirepositions and tires of other constructions.

For purposes of creating the correlation, a pressure range from 70 psito 120 psi was selected, which represents a range from 5 psi less thanthe minimum pressure desired P_(min-lowest)=75 psi for this tire to themaximum recommended pressure for this tire. Increments of about 5 psiwere used for creating the correlation. A load range was also selectedthat spanned from an unloaded trailer (“0%”), through the maximum legalaxle load (“100%”) allowed under the laws of the United States, and upto the maximum legal side wall load (“131%”). This load range wasdivided into 14 approximately equal increments. The loads and pressuresused are set forth in Table 1.

TABLE 1 Pressure Load PSI kPa Bar % load DaN Kg lbs 120 830 8.3 131.3%  4719 4625 10196 115 790 7.9 120%  4382 4294 9467 110 760 7.6 110%  40844002 8823 104 720 7.2 100%  3785 3709 8178 100 690 6.9 90% 3487 34177533 96 660 6.6 80% 3188 3124 6888 90 620 6.2 70% 2890 2832 6243 86 5905.9 60% 2591 2539 5598 80 550 5.5 50% 2293 2247 4953 75 520 5.2 40% 19941954 4308 70 480 4.8 30% 1696 1662 3663 20% 1397 1369 3018 10% 1099 10772373  0% 800 784 1728

For each tire load and air pressure, the longitudinal stresses werecalculated using finite element analysis under free rolling conditions.FIG. 1 provides an example of the longitudinal stresses that werecalculated over the nine ribs for a load of 3709 kilograms.

Using a threshold longitudinal stress σ_(x-th) of −0.11 bar, theexemplary method described above was applied to arrive at a data set ofpreferred pressures P_(p) for the selected range of loads. A plot of thepreferred pressures P_(p) against the selected range of loads is setforth in FIG. 2. As shown therein, for the purposes of optimizing treadwear, the preferred pressure P_(p) curve (illustrated with a straightline in FIG. 2 as the “Sx Criteria” curve) varies significantly from theair pressures recommended by the Tire and Rim Association load inflationtable (illustrated with a dashed line in FIG. 2 as the “T&RA loadpressure” curve). More specifically, the preferred pressure P_(p) curveis shifted to higher pressures and has a higher slope (2.60×10⁻³ bar/Kg)than the T&RA curve (2.14×10⁻³ bar/Kg). When the trailer load is above60 percent, the preferred pressure curve P_(p) recommends substantiallyhigher pressures, up to 2.1 bars.

While the present subject matter has been described in detail withrespect to specific exemplary embodiments and methods thereof, it willbe appreciated that those skilled in the art, upon attaining anunderstanding of the foregoing may readily produce alterations to,variations of, and equivalents to such embodiments. Accordingly, thescope of the present disclosure is by way of example rather than by wayof limitation, and the subject disclosure does not preclude inclusion ofsuch modifications, variations and/or additions to the present subjectmatter as would be readily apparent to one of ordinary skill in the art.

What is claimed is:
 1. A method for regulating tire air pressure, thetire having shoulders, a tread with a ground contacting portion, anddefining lateral and longitudinal directions, the lateral directionbeing parallel to an axis of rotation of the tire, the longitudinaldirection being perpendicular to the axis of rotation of the tire, themethod comprising the steps of: dividing the ground contacting portionof the tread along the lateral direction into multiple zones positionedadjacent to each other along the lateral direction, the multiple zonesincluding shoulder zone with each shoulder zone positioned along arespective shoulder of the tire, the multiple zones including one ormore intermediate zones between the shoulder zones; selecting a range oftire air pressures and a range of tire loads; determining, for a loadL_(i) in the selected range of tire loads, the average longitudinalstress σ_(x) in each of the zones over the selected range of tire airpressures; identifying, from the selected range of tire air pressures, apreferred air pressure P_(p) for the load L_(i), wherein the preferredair pressure P_(p) is the pressure at which both i) the averagelongitudinal stress σ_(x-avg) for all of the zones is minimized and ii)the average longitudinal stress σ_(x-avg) for each of the intermediatezones is maintained at or above a predetermined threshold longitudinalstress σ_(x-th); repeating, over the selected range of tire loads L_(i),said steps of determining and identifying so as to provide a correlationof the preferred air pressures P_(p) corresponding to multiple loadsL_(i) for the selected range of tire loads; ascertaining an actual oranticipated tire load L_(a) on a vehicle; and adjusting tire airpressure during vehicle use to the preferred air pressure P_(p) for thetire load L_(a) using the correlation from said step of repeating.
 2. Amethod for regulating tire air pressure as in claim 1, wherein said stepof identifying comprises, for the load L_(i): eliminating from theselected range of air pressures all pressures at which the averagelongitudinal stress σ_(x-avg) for any zone is less than the thresholdlongitudinal stress σ_(x-th) and, if there is no pressure at which theaverage longitudinal stress σ_(x-avg) for any zone is less than thethreshold longitudinal stress σ_(x-th), then eliminating from theselected range of air pressures all pressures at which the averagelongitudinal stress σ_(x-avg) for either of the shoulder zones is lessthan the threshold longitudinal stress σ_(x-th); calculating S, the sumof the squares of the average longitudinal stress σ_(x-avg), of each ofthe zones, for each of the pressures remaining in the selected range ofair pressures after said step of eliminating; and selecting, from thepressures remaining in the selected range of air pressures after saidstep of eliminating, the preferred air pressure P_(p) as the greater ofeither a) the air pressure having the smallest value of S or b) the airpressure P_(min) that is the minimum recommended for the load L_(i). 3.A method for regulating tire air pressure as in claim 2, furthercomprising the steps of: identifying the highest load L_(Pmin-lowest)for which the preferred pressure P_(p) is equal to the minimum pressuredesired P_(min-lowest) for the tire and, if such a load exists, thenselecting the minimum pressure desired P_(min-lowest) as the preferredpressures P_(p) for all loads below L_(Pmin-lowest).
 4. A method forregulating tire air pressure as in claim 1, wherein the shoulder zonesare defined to include shoulder ribs located along the shoulders of thetire.
 5. A method for regulating tire air pressure as in claim 3,wherein the one or more intermediate zones are each defined as one ofthe ribs located between the shoulder ribs of the tire.
 6. A method forregulating tire air pressure as in claim 1, wherein the selected rangeof air pressures is defined as the pressures between about 5 psi lessthan the minimum desired air pressure P_(min-lowest) for the tire toabout the maximum sidewall pressure for the tire.
 7. A method forregulating tire air pressure as in claim 1, wherein the selected rangeof loads is defined as the loads between about the minimum operatingload of the tire to about the maximum sidewall load of the tire.
 8. Amethod for regulating tire air pressure as in claim 1, wherein said stepof determining comprises measuring longitudinal stresses σ_(x) in one ormore of the zones.
 9. A method for regulating tire air pressure as inclaim 1, wherein said step of determining comprises calculating thelongitudinal stresses σ_(x) in one or more of the zones.
 10. A methodfor regulating tire air pressure as in claim 1, wherein said step ofdetermining comprises measuring or calculating the net force of contactper zone and dividing by the area of the zone in contact with theground.
 11. A method for regulating tire air pressure as in claim 1,wherein said step of adjusting the tire pressure comprises increasingthe air pressure, decreasing the air pressure, or both.
 12. A method forregulating tire air pressure as in claim 1, wherein said step ofascertaining the actual load comprises measuring the load carried by thevehicle.
 13. A method for regulating tire air pressure as in claim 1,further comprising the step of measuring tire air pressure duringvehicle use before said step of adjusting.
 14. A method for regulatingtire air pressure as in claim 1, wherein the tire is a non-ribbed tire.15. A method for regulating tire air pressure as in claim 1, wherein thetire is used on a trailer and the threshold longitudinal stress σ_(x-th)is in the range of about −0.10 bar to about 0.12 bar.
 16. A method forregulating tire air pressure as in claim 1, wherein the tire is used ina steer position and the threshold longitudinal stress σ_(x-th) is inthe range of about −0.04 bar to about −0.06 bar.